Holographic recording medium with control of photopolymerization and dark reactions

ABSTRACT

The present invention relates to a system, as well as articles and holographic recording medium comprising the system, where the system comprises: a polymerizable component comprising at least one photoactive polymerizable material; and a photoinitiator component comprising at least one photoinitiator for causing the polymerizable component to polymerize to thereby form a plurality of holographic gratings when activated by exposure to a photoinitiating light source; wherein when a portion of the polymerizable component has been polymerized to form at least one holographic grating, the unpolymerized portion of the polymerizable component is resistant to further polymerization when not exposed to the photoinitiating light source. The present invention also provides methods for forming at least one holographic grating in a holographic recording medium having such a photopolymerizable system.

BACKGROUND

1. Field of the Invention

The present invention relates to a holographic recording medium.

2. Related Art

Developers of information storage devices and methods continue to seekincreased storage capacity. As part of this development, so-calledpage-wise memory systems, in particular holographic systems, have beensuggested as alternatives to conventional memory devices. Page-wisesystems involve the storage and readout of an entire two-dimensionalrepresentation, e.g., a page of data. Typically, recording light passesthrough a two-dimensional array of dark and transparent areasrepresenting data, and the holographic system stores, in threedimensions, holographic representations of the pages as patterns ofvarying refractive index imprinted into a storage medium. Holographicsystems are discussed generally in Psaltis et al., “HolographicMemories,” Scientific American, November 1995.

One method of holographic storage is phase correlation multiplexholography, which is described in U.S. Pat. No. 5,719,691 (Curtis etal.), issued Feb. 17, 1998. In one embodiment of phase correlationmultiplex holography described in Curtis et al., a reference light beamis passed through a phase mask, and intersected in the recording mediumwith a signal beam that has passed through an array representing data,thereby forming a hologram in the medium. The spatial relation of thephase mask and the reference beam is adjusted for each successive pageof data, thereby modulating the phase of the reference beam and allowingthe data to be stored at overlapping areas in the medium. The data islater reconstructed by passing a reference beam through the originalstorage location with the same phase modulation used during datastorage. It is also possible to use volume holograms as passive opticalcomponents to control or modify light directed at the medium, e.g.,filters or beam steerers. Writing processes that provide refractiveindex changes are also capable of forming articles such as waveguides.

The capabilities of typical holographic recording systems are determinedin part by the storage medium. One type of holographic recording mediaused recently for such systems are photosensitive polymer films. See,e.g., Smothers et al., “Photopolymers for Holography,” SPIE OE/LaserConference, 1212-03, Los Angeles, Calif., 1990. The holographicrecording media described in this article contain a photoimageablesystem containing a liquid monomer material (the photoactive monomer)and a photoinitiator (which promotes the polymerization of the monomerupon exposure to light), where the photoimageable system is in anorganic polymer host matrix that is substantially inert to the exposurelight. During writing (recording) of information into the material (bypassing recording light through an array representing data), the monomerpolymerizes in the exposed regions. Due to the lowering of the monomerconcentration caused by the polymerization, monomer from the dark,unexposed regions of the material diffuses to the exposed regions. Thepolymerization and resulting diffusion create a refractive index change,thus forming the hologram (holographic grating) representing the data.

Generally, in photopolymer systems used in conventional applicationssuch as coatings, sealants, adhesives, etc., properties such as chainlength and degree of polymerization are usually maximized and driven tocompletion by using very high light intensities, multifunctionalmonomers, high concentrations of monomers, heat, etc. Similarly, priorholographic recording media have used formulations that are higher inmonomer concentration (as in typical photopolymer formulations) toprovide holographic recording media based on organic photopolymersystems. See, for example, U.S. Pat. No. 5,874,187 (Colvin et al.),issued Feb. 23, 1999, and U.S. Pat. No. 5,759,721 (Dhal et al.), issuedJun. 2, 1998, which disclose what are often referred to as“one-component” organic photopolymer systems. Such one-component systemstypically have large Bragg detuning values if they are not precured withlight to some extent. Further improvements in holographic photopolymermedia have also been made by separating the formation of the polymericmatrix from the photochemistry used to record holographic information.See, for example, U.S. Pat. No. 6,103,454 (Dhar et al.), issued Aug. 15,2000, and commonly assigned, U.S. Pat. No. 6,482,551 (Dhar et al.),issued Nov. 19, 2002, which disclose what are often referred to as“two-component” organic photopolymer systems. Two-component organicphotopolymer systems allow for more uniform starting conditions (i.e.,regarding the recording process), more convenient processing andpackaging options, and the ability to obtain very high dynamic rangemedia with very little shrinkage or Bragg detuning.

Thus, even though large improvements in holographic media have beenmade, further improvements in such media would be desirable to: (1)preserve the specified pattern of holographic gratings in the media toallow for reliable retrieval of the recorded data in such media; (2)allow for appropriate scheduling in forming holographic gratings in themedia; (3) more accurately determine the time needed for recordingadditional holographic gratings in the same volume of previouslyrecorded media, up to the full dynamic range thereof; and (4) ingeneral, to create more commercially viable high density holographicdata storage media.

SUMMARY

According to a first broad aspect of the present invention, there isprovided a system comprising:

-   -   a polymerizable component comprising at least one photoactive        polymerizable material; and    -   a photoinitiator component comprising at least one        photoinitiator for causing the polymerizable component to        polymerize to thereby form at least one holographic grating when        activated by exposure to a photoinitiating light source;    -   wherein when a portion of the polymerizable component has been        polymerized to form at least one holographic grating, the        unpolymerized portion of the polymerizable component is        resistant to further polymerization when the polymerizable        component is not exposed to the photoinitiating light source.

According to a second broad aspect of the present invention, there isprovided an article comprising a support matrix and a photopolymerizablesystem in the support matrix, the photopolymerizable system comprising:

-   -   a polymerizable component comprising at least one photoactive        polymerizable material; and    -   a photoinitiator component comprising at least one        photoinitiator for causing the polymerizable component to        polymerize to thereby form at least one holographic grating in        the support matrix when activated by exposure to a        photoinitiating light source;    -   wherein when a portion of the polymerizable component has been        photopolymerized to form at least one holographic grating, the        unpolymerized portion of the polymerizable component is        resistant to further polymerization when not exposed to the        photoinitiating light source.

According to a third broad aspect of the present invention, there isprovided a method comprising the following steps:

-   -   (a) providing at least one holographic recording medium; and    -   (b) forming at least one holographic grating in the holographic        recording medium, wherein the holographic recording medium has a        photopolymerizable system comprising:    -   a polymerizable component comprising at least one photoactive        polymerizable material; and    -   a photoinitiator component comprising at least one        photoinitiator for causing the polymerizable component to        polymerize to thereby form a plurality of holographic gratings        in the holographic recording medium when activated by exposure        to recording light;    -   wherein the unpolymerized portion of the polymerizable component        is resistant to further polymerization when not exposed to the        recording light.

According to a fourth broad aspect of the present invention, there isprovided a method comprising the steps of:

(a) providing a holographic recording medium; and

(b) forming a plurality of holographic gratings in the holographicrecording medium, wherein each holographic grating in the holographicrecording medium is formed by exposing the holographic recording mediumto recording light according to a schedule that is a function of whenthe holographic recording medium was exposed to the recording lightversus the time period of each exposure to the recording light, andwherein the holographic recording medium has a photopolymerizable systemcomprising:

-   -   a polymerizable component comprising at least one photoactive        polymerizable material; and    -   a photoinitiator component comprising at least one        photoinitiator for causing the polymerizable component to        polymerize to thereby form a plurality of holographic gratings        in the holographic recording medium when activated by exposure        to recording light;    -   wherein the unpolymerized portion of the polymerizable component        is resistant to further polymerization when not exposed to the        recording light.

According to a fifth broad aspect of the present invention, there isprovided a method comprising the following steps:

(a) providing a holographic recording medium having therein one or morefirst holographic gratings; and

(b) forming one or more additional holographic gratings in theholographic recording medium, wherein the holographic recording mediumhas a photopolymerizable system comprising:

-   -   an unpolymerized portion of a polymerizable component comprising        at least one photoactive polymerizable material; and    -   a photoinitiator component comprising at least one        photoinitiator for causing the unpolymerized portion of the        polymerizable component to polymerize to thereby form at least        one additional holographic grating in the holographic recording        medium when activated by exposure to recording light;    -   wherein the unpolymerized portion of the polymerizable component        is resistant to further polymerization when not exposed to the        recording light.

According to an sixth broad aspect of the present invention, there isprovided a method comprising the following steps:

(a) providing a recording medium having therein a plurality of firstholographic gratings in a volume thereof; and

(b) forming a plurality of additional holographic gratings in theholographic recording medium in the volume of the holographic recordingmedium, wherein the plurality of additional holographic gratings areformed according to a schedule that is a function of when eachadditional holographic grating is formed versus the time period taken toform each additional holographic grating, and wherein the holographicmedium has a photopolymerizable system comprising:

-   -   an unpolymerized portion of a polymerizable component comprising        at least one photoactive polymerizable material; and    -   a photoinitiator component comprising at least one        photoinitiator for causing the polymerizable component of the        unpolymerized portion to polymerize to thereby form a plurality        of additional holographic gratings in the holographic recording        medium when activated by exposure to recording light;    -   wherein the unpolymerized portion of the polymerizable component        is resistant to further polymerization when not exposed to the        recording light.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the accompanyingdrawings, in which:

FIG. 1 shows a basic holographic storage system according to the presentinvention;

FIG. 2 is an image of a single holographic data page recorded in atwo-component photopolymerization medium that contains residualunpolymerized monomer and unused photoinitiator;

FIG. 3 shows two graphical plots, one of the light intensity in totalcounts per millisecond recorded, the other of the signal to noise ratio(SNR), as a function of elapsed time, for the data page of FIG. 2;

FIG. 4 is a histogram showing the relative fraction of pixels having aparticular light intensity for the data page of FIG. 2;

FIG. 5 is an image mapping the local signal to noise ratio (SNR) for thedata page of FIG. 2, (8×8 data blocks);

FIG. 6 is an image of the holographic data page of FIG. 2 after being inthe dark (i.e., without exposure to recording light) for 60 minutes;

FIG. 7 shows two graphical plots, one of the light intensity in totalcounts per millisecond recorded, the other of the signal to noise ratio(SNR), as a function of elapsed time, for the data page of FIG. 6;

FIG. 8 is a histogram showing the relative fraction of pixels having aparticular light intensity for the data page of FIG. 6; and

FIG. 9 shows two graphical plots of the signal to noise ratio (SNR) of adata page as a function of time, one with no polymerizationretarder/inhibitor added, the other with 0.1% of a polymerizationretarder/inhibitor (BHT) added.

DETAILED DESCRIPTION

It is advantageous to define several terms before describing theinvention. It should be appreciated that the following definitions areused throughout this application.

DEFINITIONS

Where the definition of terms departs from the commonly used meaning ofthe term, applicant intends to utilize the definitions provided below,unless specifically indicated.

For the purposes of the present invention, the term “light source”refers to any source of electromagnetic radiation of any wavelength. Inone embodiment, the light source of the present invention is a laser ofa particular wavelength.

For the purposes of the present invention, the term “photoinitiatinglight source” refers to a light source that activates a photoinitiator,a photoactive polymerizable material, or both. Photoiniating lightsources include recording light, etc.

For the purposes of the present invention, the term “spatial lightintensity” refers to a light intensity distribution or patterns ofvarying light intensity within a given volume of space.

For the purposes of the present invention, the term “holographicgrating” or “hologram” are used in the conventional sense of referringto a recorded interference pattern formed when a signal beam and areference beam interfere with each other. In cases where in digital datais recorded, the signal beam is encoded with a spatial light modulator.

For the purposes of the present invention, the term “holographicrecording” refers to a holographic grating after it is recorded in theholographic recording medium.

For the purposes of the present invention, the term “holographicrecording medium” refers to an article that is capable of recording andstoring, in three dimensions, one or more holographic gratings as one ormore pages as patterns of varying refractive index imprinted into anarticle.

For the purposes of the present invention, the term “data page” or“page” refers to the conventional meaning of data page as used withrespect to holography. For example, a data page may be a page of data,one or more pictures, etc., to be recorded in a holographic recordingmedium, such as an article of the present invention.

For the purposes of the present invention, the term “recording light”refers to a light source used to record into a holographic medium. Thespatial light intensity pattern of the recording light is what isrecorded. Thus, if it is a simple noncoherent beam of light then awaveguide may be created, or if it is two interfering laser beams, theninterference patterns will be recorded.

For the purposes of the present invention, the term “recording data”refers to storing holographic representations of one or more pages aspatterns of varying refractive index.

For the purposes of the present invention, the term “reading data”refers to retrieving data stored as holographic representations.

For the purposes of the present invention, the term “exposure” refers towhen a holographic recording medium was exposed to recording light,i.e., when the holographic grating was recorded in the medium.

For the purposes of the present invention, the terms “time period ofexposure” and “exposure time” refer interchangeably to how long theholographic recording medium was exposed to recording light, i.e., howlong the recording light was on during the recording of a holographicgrating in the holographic recording medium. “Exposure time” can referto the time required to record a single hologram or the cumulative timefor recording a plurality of holograms in a given volume.

For the purposes of the present invention, the term “schedule” refers tothe pattern, plan, scheme, sequence, etc., of the exposures relative tothe cumulative exposure time in recording holographic gratings in amedium. In general, the schedule allows one to predict the time (orlight energy) needed for each single exposure, in a set of pluralexposures, to give a predetermined diffraction efficiency.

For the purposes of the present invention, the term “function” when usedwith the term “schedule” refers to a graphical plot or mathematicalexpression that defines or describes a schedule of exposures versuscumulative exposure time in recording plural holographic gratings.

For the purposes of the present invention, the term “substantiallylinear function” when used with the term “schedule” refers to agraphical plot of the schedule of exposures versus exposure time thatprovides a straight line or substantially a straight line.

For the purposes of the present invention, the term “support matrix”refers to the material, medium, substance, etc., in which thepolymerizable component is dissolved, dispersed, embedded, enclosed,etc. The support matrix is typically a low T_(g) polymer. The polymermay be organic, inorganic, or a mixture of the two. The polymer may alsobe either a thermoset or thermoplastic.

For the purposes of the present invention, the term “different form”refers to an article of the present invention being processed to form aproduct having a different form such as processing an article comprisinga block of material, powder of material, chips of material, etc. into amolded product, a sheet, a free flexible film, a stiff card, a flexiblecard, an extruded product, a film deposited on a substrate, etc.

For the purposes of the present invention, the term “particle material”refers to a material that is made by grinding, shredding, fragmenting orotherwise subdividing an article into smaller components or to amaterial that is comprised of small components such as a powder.

For the purposes of the present invention, the term “free flexible film”refers to a thin sheet of flexible material that maintains its formwithout being supported on a substrate. Examples of free flexible filmsinclude the various types of plastic wraps used in food storage.

For the purposes of the present invention, the term “stiff article”refers to an article that may crack or crease when bent. An example of astiff article is a plastic credit card, a DVD, a transparency, wrappingpaper, a shipping box, etc.

For the purposes of the present invention, the term “volatile compound”refers to any chemical with a high vapor pressure and/or a boiling pointbelow about 150° C. Examples of volatile compounds include: acetone,methylene chloride, toluene, etc. An article, mixture or component is“volatile compound free” if the article, mixture or component does notinclude a volatile compound.

For the purposes of the present invention, the term “oligomer” refers toa polymer having approximately 30 repeat units or less or any largemolecule able to diffuse at least about 100 nm in approximately 2minutes at room temperature when dissolved in the article of the presentinvention. Such oligomers may contain one or more polymerizable groupswhereby the polymerizable groups may be the same or different from otherpossible monomers in the polymerizable component. Furthermore, when morethan one polymerizable group is present on the oligomer, they may be thesame or different. Additionally, oligomers may be dendritic. Oligomersare considered herein to be photoactive monomers, although they aresometimes referred to as “photoactive oligomer(s)”.

For the purposes of the present invention, the term“photopolymerization” refers to any polymerization reaction caused byexposure to a photoinitiating light source.

For the purposes of the present invention, the term “resistant tofurther polymerization” refers to the unpolymerized portion of thepolymerizable component having a deliberately controlled andsubstantially reduced rate of polymerization when not exposed to aphotoiniating light source such that dark reactions are minimized,reduced, diminished, eliminated, etc. A substantial reduction in therate of polymerization of the unpolymerized portion of the polymerizablecomponent according to the present invention can be achieved by anysuitable composition, compound, molecule, method, mechanism, etc., orany combination thereof, including using one or more of the following:(1) a polymerization retarder; (2) a polymerization inhibitor; (3) achain transfer agent; (4) metastable reactive centers; (5) a light orheat labile phototerminator; (6) photo-acid generators, photo-basegenerators or photogenerated radicals; (7) polarity or solvationeffects; (8) counter ion effects; and (9) changes in monomer reactivity.

For the purposes of the present invention, the term “substantiallyreduced rate” refers to a lowering of the polymerization rate to a rateapproaching zero, and ideally a rate of zero, within seconds after thephotoinitiating light source is off or absent. The rate ofpolymerization should typically be reduced enough to prevent the loss infidelity of previously recorded holograms.

For the purposes of the present invention, the term “dark reaction”refers to any polymerization reaction that occurs in absence of aphotoinitiating light source. Dark reactions can deplete unused monomer(loss of dynamic range), can cause noise gratings (including stray lightgratings), or can cause unpredictability in the scheduling used forrecording additional holograms.

For the purposes of the present invention, the term “free radicalpolymerization” refers to any polymerization reaction that is initiatedby any molecule comprising a free radical or radicals.

For the purposes of the present invention, the term “cationicpolymerization” refers to any polymerization reaction that is initiatedby any molecule comprising a cationic moiety or moieties.

For the purposes of the present invention, the term “anionicpolymerization” refers to any polymerization reaction that is initiatedby any molecule comprising an anionic moiety or moieties.

For the purpose of the present invention, the term “photoinitiator”refers to the conventional meaning of the term photoinitiator and alsorefers to sensitizers and dyes. In general, a photoinitiator causes thelight initiated polymerization of a material, such as a photoactiveoligomer or monomer, when the material containing the photoinitiator isexposed to light of a wavelength that activates the photoinitiator,i.e., a photoinitiating light source. The photoinitiator may refer to acombination of components, some of which individually are not lightsensitive, yet in combination are capable of curing the photoactiveoligomer or monomer, examples of which include a dye/amine, asensitizer/iodonium salt, a dye/borate salt, etc.

For the purposes of the present invention, the term “photoinitiatorcomponent” refers to a single photoinitiator or a combination of two ormore photoinitiators. For example, two or more photoinitiators may beused in the photoinitiator component of the present invention to allowrecording at two or more different wavelengths of light.

For the purposes of the present invention the term “polymerizablecomponent” refers to a mixture of one or more photoactive polymerizablematerials, and possibly one or more additional polymerizable materials(i.e., monomers and/or oligomers) that are capable of forming a polymer.

For the purposes of the present invention, the term “photoactivepolymerizable material” refers to a monomer, an oligomer andcombinations thereof that polymerize in the presence of a photoinitiatorthat has been activated by being exposed to a photoinitiating lightsource, e.g., recording light. In reference to the functional group thatundergoes curing, the photoactive polymerizable material comprises atleast one such functional group. It is also understood that there existphotoactive polymerizable materials that are also photoinitiators, suchas N-methylmaleimide, derivatized acetophenones, etc. In such a case, itis understood that the photoactive monomer and/or oligomer of thepresent invention may also be a photoinitiator.

For the purposes of the present invention, the term “photopolymer”refers to a polymer formed by one or more photoactive polymerizablematerials, and possibly one or more additional monomers and/oroligomers.

For the purposes of the present invention, the term “polymerizationretarder” refers to one or more compositions, compounds, molecules,etc., that are capable of slowing, reducing, etc., the rate ofpolymerization while the photoinitiating light source is off or absent,or inhibiting the polymerization of the polymerizable component when thephotoinitiating light source is off or absent. A polymerization retarderis typically slow to react with a radical (compared to an inhibitor),thus while the photoinitiating light source is on, polymerizationcontinues at a reduced rate because some of the radicals are effectivelyterminated by the retarder. However, at high enough concentrations, apolymerization retarder can potentially behave as a polymerizationinhibitor. For the purposes of the present invention, it is desirable tobe within the concentration range that allows for retardation ofpolymerization to occur, rather than inhibition of polymerization.

For the purposes of the present invention, the term “polymerizationinhibitor” refers to one or more compositions, compounds, molecules,etc., that are capable of inhibiting or substantially inhibiting thepolymerization of the polymerizable component when the photoinitiatinglight source is on or off. Polymerization inhibitors typically reactvery quickly with radicals and effectively stop a polymerizationreaction. Inhibitors cause an inhibition time during which little to nophotopolymer forms (i.e., only very small chains). Typically,photopolymerization occurs only after nearly 100% of the inhibitor isreacted.

For the purposes of the present invention, the term “chain transferagent” refers to one or more compositions, compounds, molecules, etc.that are capable of interrupting the growth of a polymeric molecularchain by formation of a new radical that may react as a new nucleus forforming a new polymeric molecular chain. Typically, chain transferagents cause the formation of a higher proportion of shorter polymerchains, relative to polymerization reactions that occur in the absenceof chain transfer agents. Additionally, traditional chain transferagents can behave as retarders or inhibitors if they do not efficientlyreinitiate polymerization.

For the purposes of the present invention, the term “metastable reactivecenters” refers to one or more compositions, compounds, molecules, etc.,that have the ability to create pseudo-living radical polymerizationswith certain polymerizable components. It is also understood thatinfrared light or heat may be used to activate metastable reactivecenters towards polymerization.

For the purposes of the present invention, the term “light or heatlabile phototerminators” refers to one or more compositions, compounds,components, materials, molecules, etc., capable of undergoing reversibletermination reactions using a light source and/or heat.

For the purposes of the present invention, the terms “photo-acidgenerators” “photo-base generators” and “photogenerated radicals” referto one or more compositions, compounds, molecules, etc., that, whenexposed to a light source, generate one or more compositions, compounds,molecules, etc., that are acidic, basic or a free radical.

For the purposes of the present invention, the term “polarity orsolvation effects” refers to an effect or effects that the solvent orthe polarity of the medium has on the polymerization rate. This effectis most pronounced for ionic polymerizations wherein the proximity ofthe counter ion to the reactive chain end influences the polymerizationrate.

For the purposes of the present invention, the term “counter ioneffects” refers to the effect that counter ion, in ionicpolymerizations, has on the kinetic chain length. Good counter ionsallow for very long kinetic chain lengths, whereas poor counter ionstend to collapse with the reactive chain end, thus terminating thekinetic chain (i.e., causing smaller chains to be formed).

For the purposes of the present invention, the term “plasticizer” refersto the conventional meaning of the term plasticizer. In general, aplasticizer is a compound added to a polymer both to facilitateprocessing and to increase the flexibility and/or toughness of a productby internal modification (solvation) of a polymer molecule.

For the purposes of the present invention, the term “thermoplastic”refers to the conventional meaning of thermoplastic, i.e., acomposition, compound, substance, etc., that exhibits the property of amaterial, such as a high polymer, that softens when exposed to heat andgenerally returns to its original condition when cooled to roomtemperature. Examples of thermoplastics include, but are not limited to:poly(methyl vinyl ether-alt-maleic anhydride), poly(vinyl acetate),poly(styrene), poly(propylene), poly(ethylene oxide), linear nylons,linear polyesters, linear polycarbonates, linear polyurethanes, etc.

For the purposes of the present invention, the term “room temperaturethermoplastic” refers to a thermoplastic that is solid at roomtemperature, i.e., will not cold flow at room temperature.

For the purposes of the present invention, the term “room temperature”refers to the commonly accepted meaning of room temperature.

For the purposes of the present invention, the term “thermoset” refersto the conventional meaning of thermoset, i.e., a composition, compound,substance, etc., that is crosslinked such that it does not have amelting temperature. Examples of thermosets are crosslinkedpoly(urethanes), crosslinked poly(acrylates), crosslinked poly(styrene),etc.

Description

The present invention is based on the discovery that a holographicrecording medium using photopolymerization to record data thereinsuffers from “dark reactions.” These “dark reactions” can continue longafter the recording of data in the medium has stopped, e.g., after therecording light has been shut off, and are independent of the particularpolymerization mechanism used, e.g., these reactions can occur in freeradical, cationic, and anionic initiated and catalyzedphotopolymerizations, etc. Over a relatively short period of time (e.g.,within as little as 5 minutes after the recording light has been shutoff), these “dark reactions” can cause noise gratings to grow in aholographic recording medium, eventually obscuring the previouslyrecorded data. Because the desired pattern of holographic gratingsrecorded in the medium has been altered by these “dark reactions,” themedium can become unusable and unreliable for retrieval of the recordeddata.

The present invention is additionally based on the discovery that these“dark reactions” can complicate the desired scheduling of holographicrecording in the medium. When recording numerous holograms in a givenvolume of material, it is desirable to record holograms of similardiffraction efficiency. To provide high density holographic datastorage, a large number of relatively weak holographic images,(diffraction efficiency<<1%) are typically recorded. This can beachieved by varying the amount of exposure time for each holographicgrating that is recorded. The schedule for recording such holographicgratings is also desirably a substantially linear function of the numberof exposures (i.e., when the holographic grating was formed) versus thetime period of each exposure (i.e., how long the recording light was onduring the formation of the holographic grating).

These “dark reactions” can create undesired nonlinear dependencies insuch scheduling. Such nonlinear scheduling is caused by at least twovariables, namely, the time between exposures and the initial dynamicrange of the medium. For holographic data storage to be commerciallyviable, it is sometimes necessary to record several holographic gratingsin a volume of the medium, and then sometime later record additionalholographic gratings in the same volume. “Dark reactions” can prevent ormake extremely difficult the accurate determination of the time neededfor recording additional holographic gratings for efficient and fullutilization of the dynamic range of the medium. The larger the originaldynamic range of the medium, the more pronounced this nonlinearscheduling problem has been found to become. Indeed, these “darkreactions” can consume a portion, up to all of the remaining dynamicrange of the medium, thus reducing or preventing the ability to recordadditional holographic gratings therein.

The present invention is further based on the discovery that these “darkreactions” are caused by the lack of termination of reactions when therecording light source is turned off. Related to this phenomenon, straylight can lead to uncontrolled polymerization, even when no longerexposed to a source of photoinitiating light (e.g., recording light).This uncontrolled polymerization has been found to cause the developmentof stray light gratings in the holographic recording medium thateventually alter and obscure the desired pattern of holographic gratingsrecorded in the medium. This type of “dark reaction” caused by straylight (such as from substrate reflections) during recording can use up aportion or all of the remaining dynamic range of the volume of themedium, thus reducing or eliminating the ability to record additionalholographic gratings therein.

The present invention solves these “dark reaction” problems bydeliberately controlling and substantially retarding, reducing,diminishing, eliminating, etc., the further polymerization of theresidual unpolymerized photoactive material when the photoinitiatinglight source is off or absent. In addition to preserving the previouslyrecorded pattern of holographic gratings in the medium for reliableretrieval of the recorded data, the present invention can also: (1)preserve more of the remaining dynamic range of the medium for recordingadditional holographic gratings; (2) record holographic gratings in themedium according to a more easily managed schedule, e.g., one that is asubstantially linear function of when the holographic recording mediumis exposed to the recording light versus the time period of eachexposure to the recording light; (3) determine more accurately the timeneeded for recording additional holographic gratings in the same volumeof a previously recorded medium, up to the full dynamic range thereof;and (4) in general, to create a more commercially viable high densityholographic data storage medium.

FIG. 1 illustrates the basic components of a holographic system 10.System 10 contains a modulating device 12, a photorecording medium 14,and a sensor 16. Modulating device 12 is any device capable of opticallyrepresenting data in two-dimensions. Device 12 is typically a spatiallight modulator that is attached to an encoding unit that encodes dataonto the modulator. Based on the encoding, device 12 selectively passesor blocks portions of a signal beam 20 passing through device 12. Inthis manner, beam 20 is encoded with a data image. The image is storedby interference of the encoded signal beam 20 with a reference beam 22at a location on or within photorecording medium 14. The interferencecreates an interference pattern (or hologram) that is captured withinmedium 14 as a pattern of, for example, varying refractive index. It ispossible for more than one holographic image to be stored at a singlelocation, or for holograms to be stored in overlapping positions, by,for example, varying the angle, the wavelength, or the phase of thereference beam 22, depending on the particular reference beam employed.Signal beam 20 typically passes through lens 30 before being intersectedwith reference beam 22 in medium 14. It is possible for reference beam22 to pass through lens 32 before this intersection. Once data is storedin medium 14, it is possible to retrieve the data by intersectingreference beam 22 with medium 14 at the same location and at the sameangle, wavelength, or phase at which reference beam 22 was directedduring storage of the data. The reconstructed data passes through lens34 and is detected by sensor 16. Sensor 16 is, for example, a chargecoupled device or an active pixel sensor. Sensor 16 typically isattached to a unit that decodes the data.

As discussed above, formation of a hologram, waveguide, or other opticalarticle relies on a refractive index contrast (Δn) between exposed andunexposed regions of a medium, this contrast being at least partly dueto monomer/oligomer diffusion to exposed regions. High index contrast isgenerally desired because it provides improved signal strength whenreading a hologram, and provides efficient confinement of an opticalwave in a waveguide. One way to provide high index contrast in theinvention is to use a photoactive monomer/oligomer having moieties(referred to as index-contrasting moieties) that are substantiallyabsent from the support matrix, and that exhibit a refractive indexsubstantially different from the index exhibited by the bulk of thesupport matrix. For example, high contrast may be obtained by using asupport matrix that contains primarily aliphatic or saturated alicyclicmoieties with a low concentration of heavy atoms and conjugated doublebonds (providing low index) and a photoactive monomer/oligomer made upprimarily of aromatic or similar high-index moieties.

A holographic recording medium of the present invention is formed suchthat holographic writing and reading to the medium are possible.Typically, fabrication of the medium involves depositing a combination,blend, mixture, etc., of the support matrix/polymerizablecomponent/photoinitiator component, as well as any composition,compound, molecule, etc., used to control or substantially reduce therate of polymerization in the absence of a photoinitiating light sourceaccording to the present invention (e.g., polymerization retarder),between two plates using, for example, a gasket to contain the mixture.The plates are typically glass, but it is also possible to use othermaterials transparent to the radiation used to write data, e.g., aplastic such as polycarbonate or poly(methyl methacrylate). It ispossible to use spacers between the plates to maintain a desiredthickness for the recording medium. In applications requiring opticalflatness, the liquid mixture may shrink during cooling (if athermoplastic) or curing (if a thermoset) and thus distort the opticalflatness of the article. To reduce such effects, it is useful to placethe article between plates in an apparatus containing mounts, e.g.,vacuum chucks, capable of being adjusted in response to changes inparallelism and/or spacing. In such an apparatus, it is possible tomonitor the parallelism in real-time by use of conventionalinterferometric methods, and to make any necessary adjustments to theheating/cooling process. Additionally, an article or substrate of thepresent invention may have an antireflective coating and/or be edgesealed to exclude water and/or oxygen. An antireflective coating may bedeposited on an article or substrate by various processes such aschemical vapor deposition and an article or substrate may be edge sealedusing known methods. The photorecording material of the presentinvention is also capable of being supported in other ways. Moreconventional polymer processing is also envisioned, e.g., closed moldformation or sheet extrusion. A stratified medium is also contemplated,i.e., a medium containing multiple substrates, e.g., glass, with layersof photorecording material disposed between the substrates.

Because the article of the present invention can, in some embodiments,exhibit thermoplastic properties, an article of the present inventionmay also be heated above its melting temperature and processed in theways described above for the combination, blend, mixture, etc., of thesupport matrix/polymerizable component/photoinitiatorcomponent/polymerization retarder.

A holographic recording medium of the present invention is then capableof being used in a holographic system such as discussed previously. Theamount of information capable of being stored in a holographic medium isproportional to the product of: the refractive index contrast, Δn, ofthe photorecording material, and the thickness, d, of the photorecordingmaterial. (The refractive index contrast, Δn, is conventionally known,and is defined as the amplitude of the sinusoidal variations in therefractive index of a material in which a plane-wave, volume hologramhas been written. The refractive index varies as: n(x)=n₀+Δn cos(K_(x)),where n(x) is the spatially varying refractive index, x is the positionvector, K is the grating wave vector, and no is the baseline refractiveindex of the medium. See, e.g., P. Hariharan, Optical Holography:Principles, Techniques and Applications, Cambridge University Press,Cambridge, 1991, at 44, the disclosure of which is hereby incorporatedby reference.) The Δn of a material is typically calculated from thediffraction efficiency or efficiencies of a single volume hologram or amultiplexed set of volume holograms recorded in a medium. The Δn isassociated with a medium before writing, but is observed by measurementperformed after recording. Advantageously, the photorecording materialof the invention exhibits a Δn of 3×10⁻³ or higher.

Examples of other optical articles include beam filters, beam steerersor deflectors, and optical couplers. (See, e.g., L. Solymar and D.Cooke, Volume Holography and Volume Gratings, Academic Press, 315-327(1981), the disclosure of which is hereby incorporated by reference.) Abeam filter separates part of an incident laser beam that is travelingalong a particular angle from the rest of the beam. Specifically, theBragg selectivity of a thick transmission hologram is able toselectively diffract light along a particular angle of incidence, whilelight along other angles travels undeflected through the hologram. (See,e.g., J. E. Ludman et al., “Very thick holographic nonspatial filteringof laser beams,” Optical Engineering, Vol. 36, No. 6, 1700 (1997), thedisclosure of which is hereby incorporated by reference.) A beam steereris a hologram that deflects light incident at the Bragg angle. Anoptical coupler is typically a combination of beam deflectors that steerlight from a source to a target. These articles, typically referred toas holographic optical elements, are fabricated by imaging a particularoptical interference pattern within a recording medium, as discussedpreviously with respect to data storage. Media for these holographicoptical elements are capable of being formed by the techniques discussedherein for recording media or waveguides.

As mentioned previously, the materials principles discussed herein areapplicable not only to hologram formation, but also to formation ofoptical transmission devices such as waveguides. Polymeric opticalwaveguides are discussed for example in B. L. Booth, “OpticalInterconnection Polymers,” in Polymers for Lightwave and IntegratedOptics, Technology and Applications, L. A. Hornak, ed., Marcel Dekker,Inc. (1992); U.S. Pat. No. 5,292,620 (Booth et al.), issued Mar. 18,1994; and U.S. Pat. No. 5,219,710 (Horn et al.), issued Jun. 15 1993,the disclosures of which are hereby incorporated by reference.Essentially, the recording material of the present invention isirradiated in a desired waveguide pattern to provide refractive indexcontrast between the waveguide pattern and the surrounding (cladding)material. It is possible for exposure to be performed, for example, by afocused laser light or by use of a mask with a non-focused light source.Generally, a single layer is exposed in this manner to provide thewaveguide pattern, and additional layers are added to complete thecladding, thereby completing the waveguide. This process is discussedfor example at pages 235-36 of Booth, supra, and Cols. 5 and 6 of U.S.Pat. No. 5,292,620, supra, the disclosure of which is herebyincorporated by reference.

In one embodiment of the present invention using conventional moldingtechniques, it is possible to mold the combination, blend, mixture,etc., of the support matrix/polymerizable component/photoinitiatorcomponent/polymerization retarder into a variety of shapes prior toformation of the article by cooling to room temperature. For example,the combination, blend, mixture, etc., of the supportmatrix/polymerizable component/photoinitiator component/polymerizationretarder can be molded into ridge waveguides, wherein a plurality ofrefractive index patterns are then written into the molded structures.It is thereby possible to easily form structures such as Bragg gratings.This feature of the present invention increases the breadth ofapplications in which such polymeric waveguides would be useful.

In one embodiment, the support matrix is thermoplastic and allows anarticle of the present invention to behave as if the entire article wasa thermoplastic. That is, the support matrix allows the article to beprocessed similar to the way that a thermoplastic is processed, i.e.,molded into a shaped article, blown into a film, deposited in liquidform on a substrate, extruded, rolled, pressed, made into a sheet ofmaterial, etc. and then allowed to harden at room temperature to take ona stable shape or form. The support matrix may comprise one or morethermoplastics. Suitable thermoplastics include poly(methyl vinylether-alt-maleic anhydride), poly(vinyl acetate), poly(styrene),poly(propylene), poly(ethylene oxide), linear nylons, linear polyesters,linear polycarbonates, linear polyurethanes, poly(vinyl chloride),poly(vinyl alcohol-co-vinyl acetate).

In one embodiment of the present invention, the amount of thermoplasticused in the holographic recording medium of the present invention ispreferably enough that the entire holographic recording mediumeffectively acts as a thermoplastic for most processing purposes. Thebinder component of the holographic recording medium may make up as muchas about 5%, preferably as much as about 50%, and more preferably asmuch as about 90% of the holographic recording medium by weight. Theamount of any given support matrix in the holographic recording mediummay vary based on clarity, refractive index, melting temperature, T_(g),color, birefringence, solubility, etc. of the thermoplastic orthermoplastics that make up the binder component. Additionally, theamount of the support matrix in the holographic recording medium mayvary based on the article's final form, whether it is a solid, aflexible film, or an adhesive.

In one embodiment of the present invention, the support matrix mayinclude a telechelic thermoplastic resin—meaning that the thermoplasticpolymer may be functionalized with reactive groups that covalentlycrosslink the thermoplastic in the support matrix with the polymerformed from the polymerizable component during grating formation. Suchcrosslinking makes the gratings stored in the thermoplastic holographicrecording medium very stable, even to elevated temperatures for extendedperiods of time.

Similarly, in another embodiment of the present invention wherein athermoset is formed, the matrix may contain functional groups thatcopolymerize or otherwise covalently bond with the monomer used to formthe photopolymer. Such matrix attachment methods allow for increasedarchival life of the recorded holograms. Suitable thermoset systems forused herein are disclosed in to U.S. Pat. No. 6,482,551 (Dhar et al.),which is incorporated here by reference.

In another embodiment of the present invention, the thermoplasticsupport matrix becomes crosslinked noncovalently with the polymer formedupon grating formation by using a functionalized thermoplastic polymerin the support matrix. Examples of such non-covalent bonding includeionic bonding, hydrogen bonding, dipole-dipole bonding, aromatic pistacking, etc.

According to an embodiment of the present invention, the polymerizablecomponent of an article of the present invention includes at least onephotoactive polymerizable material that can form holographic gratingsmade of a polymer or co-polymer when exposed to a photoinitiating lightsource, such as a laser beam that is recording data pages to theholographic recording medium. The photoactive polymerizable materialscan include any monomer, oligomer, etc., that is capable of undergoingphotoinitiated polymerization, and which, in combination with thesupport matrix, meets the compatibility requirements of the presentinvention. Suitable photoactive polymerizable materials include thosewhich polymerize by a free-radical reaction, e.g., molecules containingethylenic unsaturation such as acrylates, methacrylates, acrylamides,methacrylamides, styrene, substituted styrenes, vinyl naphthalene,substituted vinyl naphthalenes, and other vinyl derivatives.Free-radical copolymerizable pair systems such as vinyl ether/maleimide,vinyl ether/thiol, acrylate/thiol, vinyl ether/hydroxy, etc., are alsosuitable. It is also possible to use cationically polymerizable systems;a few examples are vinyl ethers, alkenyl ethers, allene ethers, keteneacetals, epoxides, etc. Furthermore, anionic polymerizable systems aresuitable. It is also possible for a single photoactive polymerizablemolecule to contain more than one polymerizable functional group. Othersuitable photoactive polymerizable materials include cyclic disulfidesand cyclic esters. Oligomers that may be included in the polymerizablecomponent to form a holographic grating upon exposure to aphotoinitiating light source include oligomers such as oligomeric(ethylene sulfide) dithiol, oligomeric (phenylene sulfide) dithiol,oligomeric (bisphenol A), oligomeric (bisphenol A) diacrylate,oligomeric polyethylene with pendent vinyl ether groups, etc. Thephotoactive polymerizable material of the polymerizable component of anarticle of the present invention may be monofunctional, difunctional,and/or multifunctional.

In addition to the at least one photoactive polymerizable material, anarticle of the present invention may contain a photoinitiator. Thephotoinitiator, upon exposure to relatively low levels of the recordinglight, chemically initiates the polymerization of the at least onephotoactive polymerizable material, thus avoiding the need for directlight-induced polymerization. The photoinitiator generally should offera source of species that initiate polymerization of the particularphotoactive polymerizable material, e.g., photoactive monomer.Typically, from about 0.1 to about 20 vol. % photoinitiator providesdesirable results.

A variety of photoinitiators known to those skilled in the art andavailable commercially are suitable for use in the present invention. Itis advantageous to use a photoinitiator that is sensitive to light atwavelengths available from conventional laser sources, e.g., the blueand green lines of Ar⁺ (458, 488, 514 nm) and He—Cd lasers (442 nm), thegreen line of frequency doubled YAG lasers (532 nm), and the red linesof He—Ne (633 nm), Kr⁺ lasers (647 and 676 nm), and various diode lasers(290 to 900 nm). One advantageous free radical photoinitiator isbis(η-5-2,4-cyclopentadien-1-yl)bis[2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl]titanium,available commercially from Ciba as Irgacure 784™. Another visiblefree-radical photoinitiator (which requires a co-initiator) is5,7-diiodo-3-butoxy-6-fluorone, commercially available from SpectraGroup Limited as H-Nu 470. Free-radical photoinitiators of dye-hydrogendonor systems are also possible. Examples of suitable dyes includeeosin, rose bengal, erythrosine, and methylene blue, and suitablehydrogen donors include tertiary amines such as n-methyl diethanolamine. In the case of cationically polymerizable components, a cationicphotoinitiator is used, such as a sulfonium salt or an iodonium salt.These cationic photoinitiator salts absorb predominantly in the UVportion of the spectrum, and are therefore typically sensitized with asensitizer or dye to allow use of the visible portion of the spectrum.An example of an alternative visible cationic photoinitiator is(η₅-2,4-cyclopentadien-1-yl) (η₆-isopropylbenzene)-iron(II)hexafluorophosphate, available commercially from Ciba as Irgacure 261.

In most embodiments, photoinitiators of the present invention aresensitive to ultraviolet and visible radiation of from about 200 nm toabout 800 nm.

An article of the present invention may also include additives such asplasticizers for altering the properties of the article of the presentinvention including the melting point, flexibility, toughness,diffusibility of the monomers, and ease of processibililty. Examples ofsuitable plasticizers include dibutyl phthalate, poly(ethylene oxide)methyl ether, N,N-dimethylformamide, etc. Plasticizers differ fromsolvents in that solvents are typically evaporated whereas plasticizersare meant to remain in the article.

Other types of additives that may be used in the liquid mixture andarticle of the present invention are inert diffusing agents havingrelatively high or low refractive indices. Inert diffusing agentstypically diffuse away from the grating being formed, and can be of highor low refractive index but are typically low. Thus, when the monomer isof high refractive index, the inert diffusing agent would be of lowrefractive index, and ideally the inert diffusing agent diffuses to thenulls in an interference pattern. Overall, the contrast of the gratingis increased. Other additives that may be used in the liquid mixture andarticle of the present invention include: pigments, fillers,nonphotoinitiating dyes, antioxidants, bleaching agents, mold releasingagents, antifoaming agents, infrared/microwave absorbers, surfactants,adhesion promoters, etc.

Although in one embodiment, the polymerizable component of an article ofthe present invention is less than about 20 volume %, in otherembodiments, the polymerizable component of an article of the presentinvention may be less than about 10 volume %, or even less than about 5volume %. For data storage applications, the typical polymerizablecomponent is present at about 5 volume %.

An article of the present invention may be any thickness needed. Forexample the article may be thin for display holography or thick for datastorage. The article may be a film deposited on a substrate, a freeflexible film (similar to food wraps) or a hard article requiring nosubstrate (similar to a credit card). For data storage applications, thearticle will typically be from about 1 to about 1.5 mm in thickness, andis typically in the form of a film deposited between two substrates withat least one of the substrates having an antireflective coating; thearticle would also likely be sealed against moisture and air.

An article of the present invention may be heated to form a liquidmixture that is infused into a porous substrate such as glass (Vycor™),cloth and paper, wood or plastic; then allowed to cool. Such articleswould be able to record holograms of a display and/or data nature.

An article of the present invention may be made optically flat via theappropriate processes, such as the process described in U.S. Pat. No.5,932,045 (Campbell et al.), issued Aug. 3, 1999, the entire contentsand disclosure of which is hereby incorporated by reference.

Also, the ability to choose between a wide variety of matrix types in anarticle of the present invention allows for the possible reduction orelimination of problems such as water or humidity that affect currentholographic storage media. In one embodiment, the article of the presentinvention may be used to store volatile holograms. Due to the ability tocontrol the photopolymer chain length in the present invention, aparticular mixture may be tuned to have a very general lifetime for therecorded holograms. Thus, after hologram recording, the holograms may bereadable for a defined time period such as a week, a few months, oryears. Heating the article may also increase such a process of hologramdestruction. Examples of applications for using volatile hologramsinclude: rental movies, security information, tickets (or seasonpasses), thermal history detector, time stamp, and/or temporary personalrecords, etc

In one embodiment, an article of the present invention may be used torecord permanent holograms. There are several methods to increase thepermanency of recorded holograms. Many of these methods involve placingfunctional groups on the matrix that allow for the attachment ofphotopolymer to the matrix during cure. The attachment groups can bevinyl unsaturations, chain transfer sites, or even a polymerizationretarder such as a BHT derivative. Otherwise, for increased archivalstability of recorded holograms, a multifunctional monomer should beused which allows for crosslinking of the photopolymer, thus increasingthe entanglement of the photopolymer in the matrix. In one embodiment ofthe present invention, both a multifunctional monomer and amatrix-attached retarder are used. In this way, the shorter chains thatare caused by the polymerization retarder do not cause loss of archivallife.

In addition to the photopolymeric systems described above, variousphotopolymeric systems may be used in the holographic recording mediumof the present invention. For example, suitable photopolymeric systemsfor use in the present invention are also described in: U.S. Pat. No.6,103,454 (Dhar et al.), U.S. Pat. No. 6,482,551 (Dhar et al.), U.S.Pat. No. 6,650,447 (Curtis et al.), U.S. Pat. No. 6,743,552(Setthachayanon et al.), U.S. Pat. No. 6,765,061 (Dhar et al.), U.S.Pat. No. 6,780,546 (Trentler et al.), U.S. Patent Application No.2003-0206320, published Nov. 6, 2003, (Cole et al), and U.S. PatentApplication No. 2004-0027625, published Feb. 12, 2004.

An article of the present invention may be ground, shredded, fragmented,etc. to form a particle material of powder, chips, etc. The particlematerial may be heated at a later time to form a flowable liquid used tomake a molded product, a coating to apply to a substrate, etc.

When the particle material is a powder, the powder may beelectrostatically applied to a substrate such as metal and otherconductive materials (typical powder coating), which would then beheated to form a coating. The powder may also be infused into fibrousmaterials to form holographic paper, cardboard, ribbons, etc. fordecorative applications. Additionally, the powder may be melt extrudedinto a fiber for thread, yarn, and fabric applications.

An article of the present invention may also be used for decorativepurposes. For example, the article may be used in gift wrap or in windowtreatments to provide special artistic tinting or 3D designs. Thearticle may be ground-up and used in coatings such as paint for houses,automobiles, furniture, etc. The article may be used in molded parts ofautomobiles, toys, furniture, appliances, etc. to provide decorativeeffects.

An article of the present invention may also be used to make datastorage devices of various sizes and shapes, as a block of material oras part of a coating that is coated on a substrate.

The present invention is particularly directed at controllingphotopolymerization reactions in the holographic recording medium, andespecially in reducing, minimizing, diminishing, eliminating, etc., darkreactions in the photopolymeric systems used in such a holographicrecording medium. This is typically achieved by using one or more of thefollowing: (1) a polymerization retarder; (2) a polymerizationinhibitor; (3) a chain transfer agent; (4) use of metastable reactivecenters; (5) use of light or heat labile phototerminators; (6) use ofphoto-acid generators, photo-base generators or photogenerated radicals;(7) use of polarity or solvation effects; (8) counter ion effects; and(9) changes in photoactive polymerizable material reactivity.

For free radical systems, the kinetics of photopolymerization reactionsare dependent on several variables such as monomer/oligomerconcentration, monomer/oligomer functionality, viscosity of the system,light intensity, photoinitiator type and concentration, the presence ofvarious additives (e.g., chain transfer agents, inhibitors), etc. Thus,for free radical photopolymerization the following steps typicallydescribe the mechanism for formation of the photopolymer:hv+PI→2 R* (initiation reaction)   1)R*+M→M* (initiation reaction)   2)M*+M→(M)₂* (propagation reaction)   3)(M)₂*+M→(M)₃* (propagation reaction)   4)(M)_(n)*+M→(M)_(n+1)* (propagation reaction)   5)R*+M*→RM (termination reaction)   6)(M)_(n)*+(M)_(m)*→(M)_(n+m) (termination reaction)   7)R*+(M)_(m)*→R(M)_(m) (termination reaction)   8)R*+R*→RR (termination reaction)   9)

The rate of photoinitiation can be described by the following equation10):R _(i) =n*Φ*I _(α) =n*Φ*ε*I ₀ *[A]*b   10)wherein R_(i) is the rate of initiation, n is the number of radicalsgenerated by the photoinitiator (n=2 for many free radical initiators,n=1 for many cationic initiators), Φ is the quantum yield for initiation(typically less than 1), I_(a) is the intensity of absorbed light inEinsteins, I₀ is incident light intensity (also in Einsteins), A is theconcentration of photoinitiator (moles/liter), C is the molarabsorptivity of the initiator at the wavelength of interest, and b isthe thickness of the system (cm).

The rate of initiation determines the rate of polymerization accordingto the following equation 11):${\left. 11 \right)\quad R_{p}} = {{k_{p}\lbrack M\rbrack}\left( \frac{\Phi\quad ɛ\quad{I_{o}\lbrack A\rbrack}b}{k_{t}} \right)^{1/n}}$wherein R_(p) is the rate of polymerization, k_(p) is the kinetic rateconstant for polymerization, M is monomer concentration, and k_(t) isthe kinetic rate constant for termination.

Equation 11) assumes that the light intensity does not vary appreciablythrough the medium. The quantum efficiency of initiation for freeradical photoinitiators is greatly affected by monomer concentration,viscosity, and rate of initiation when monomer concentration is below0.1 M, which is typically the regime for a two-component typephotopolymer holographic medium. Thus, the following dependencies arefound to decrease the quantum yield for initiation: higher viscosities,lower monomer concentration, and higher initiation rates (from increasedintensity, higher molar absorptivity, etc.).

However, when a polymerization retarder/inhibitor is added, thereactions 12) and 13) below can occur (where X* represents any radical):X*+Z-Y→X-Y+Z* (termination reaction)   12)Z*+X*→Z-X (termination reaction)   13)

Assuming that transfer to the retarder/inhibitor is high relative toother termination reactions, the kinetic equation 11) changes to thefollowing equation 14):${\left. 14 \right)\quad R_{p}} = {{k_{p}\lbrack M\rbrack}\left( \frac{\Phi\quad ɛ\quad{I_{o}\lbrack A\rbrack}b}{k_{t}\lbrack Z\rbrack} \right)}$wherein Z is the concentration of the inhibitor and k_(z) is thetermination with retarder/inhibitor rate constant. The polymerizationrate shown in equation 14) is dependent on the 1^(st) power of theinitiation rate (R_(i)). The ratio of k_(z)/k_(p) is referred to as theinhibitor constant (i.e., lower case z). Values much greater than about1 represent an inhibitory effect, whereas values of about 1 or lessrepresent retarding effects. Values much less than about 1 representlittle effect on the polymerization rate.

The difference between a polymerization inhibitor and a polymerizationretarder frequently depends on the particular polymerizable componentinvolved. For example, nitrobenzene only mildly retards radicalpolymerization of methyl acrylate, yet, nitrobenzene inhibits radicalpolymerization of vinyl acetate. Thus, it is possible to find agentsthat are typically considered as inhibitors that would also function asretarders for the purposes of the present invention. See the followingtable which shows some illustrative inhibitor constants z for variouspolymerization retarders/inhibitors with various polymer systems: MethylVinyl Acrylo- Methyl Methac- Sty- Acetate nitrile Acrylate rylate reneNitrobenzene 10 0.005 0.3 p-Benzo- 1 6 500 quinone Chloranil 0.3 2000FeCl₃ 3 540 CuCl₂ 100 1000 10,000 Aniline 0.015 0.004 0.0001 0.00040.0002 Phenol 0.012 0.0002 0.0003 0.0008 p- 1 Dihydroxy- benzene 2,4,6-0.5 trimethyl- phenol Triethyl- 0.04 3 0.04 0.0002 0.0004 amine p-cresol0.07 0.002

Suitable polymerization retarders and inhibitors for use herein includebut are not limited to one or more of the following: for free radicalpolymerizations, various phenols including butylated hydoxytoluenes(BHT) such as 2,6-di-t-butyl-p-cresol, p-methoxyphenol,diphenyl-p-benzoquinone, benzoquinone, hydroquinone, pyrogallol,resorcinol, phenanthraquinone, 2,5-toluquinone, benzylaminophenol,p-dihydroxybenzene, 2,4,6-trimethylphenol, etc.; various nitrobenzenesincluding o-dinitrobenzene, p-dinitrobenzene, m-dinitrobenzene, etc.;N-phenyl-1-naphthylamine, N-phenyl-2-naphthylamine, cupferron,phenothiazine, tannic acid, p-nitrosamine, chloranil, aniline, hinderedanilines, ferric chloride, cupric chloride, triethylamine, etc. Thesepolymerization retarders and inhibitors can be used individually (i.e.,a single retarder) or in combinations of two or more, i.e., a pluralityof retarders. Although the table above illustrates retarders andinhibitors for radical polymerization, the same principles can beapplied to ionic polymerizations. For example, it is known that chlorideanions can behave as retarders or inhibitors for cationicpolymerizations, depending on both the monomer type and theconcentration of the chloride anions. Typically, functionalities thatare basic or mildly nucleophilic behave as retarders and inhibitors forcationic polymerizations; whereas for anionic polymerizations, slightlyacidic and mildly electrophilic functionalities behave as retarders andinhibitors.

Polymerization reactions involving both polymerization retarders andinhibitors should lead to termination reactions. If reinitiation occursto any appreciable degree, then the agent is typically considered achain transfer agent. For example, triethylamine can be used as a chaintransfer agent since it is also capable of reinitiating some radicalpolymerizations; however, when the reinitiation is slow compared totermination reactions, then even chain transfer agents can be consideredpotential polymerization retarders or inhibitors for the purposes of thepresent invention.

Suitable chain transfer agents for use herein include but are notlimited to: triethylamine, thioethers, compounds having carbonategroups, ethers, toluene derivatives, allyl ethers, etc. Chain transferagents that are mildly retarding can be desirable because these can beincorporated into the matrix and enable attachment of the photopolymerand photoinitiator radicals to the matrix.

FIGS. 2 through 8 show the time dependent building of noise gratings ina radical photopolymer holographic medium that contains minimalpolymerization retarders/inhibitors. FIGS. 2 through 5 relate to asingle holographic data page recorded in a two-component radicalphotopolymer medium. As shown in FIG. 2, a high fidelity data page isrecorded in the medium. This is further demonstrated by the graphicalplots of light intensity (in total counts per millisecond recorded) andsignal to noise ratio (SNR) as a function of elapsed time in FIG. 3, thehistogram of the relative fraction of pixels having a particular lightintensity in FIG. 4, and the mapping of the SNR of FIG. 5.

FIGS. 6 through 8 represent the same medium in the dark and 60 minutesafter the recording shown in FIG. 2. As shown in FIG. 6, the medium hasdeveloped noise gratings that have grown and eventually obscured thepreviously recorded data, due to the occurrence of “dark reactions.”This degrading of the recorded data in FIG. 6 is further demonstrated bythe graphical plots of light intensity (in total counts per millisecondrecorded) and signal to noise ratio (SNR) as a function of elapsed timein FIG. 7, and the histogram of the relative fraction of pixels having aparticular light intensity in FIG. 8. Indeed, if the SNR were mapped asin FIG. 5, it would show very little difference in light intensity overthe entire data page of FIG. 6.

The graphical plots in FIG. 9 illustrate what happens when apolymerization retarder such as BHT (2,6-di-i-butyl-p-cresol)) isincluded in the holographic recording medium according to the presentinvention in amounts sufficient to retard the polymerization of thephotoactive polymerizable monomer when not exposed to a photoinitiatinglight source. The curve with squares in FIG. 9 represents the additionof no polymerization retarder, while the curve with triangles representsthe addition of 0.1% BHT. As shown in FIG. 9, when no polymerizationretarder was added, the reconstruction of the recorded page rapidlydegraded due to dark reactions, as evidenced by the greater than 30 dBdrop in SNR. By contrast, as also shown in FIG. 9, the addition of 0.1%BHT minimized or eliminated noise development due to dark reactions,allowing the recorded page to maintain high fidelity. In other words,the observed time dependent noise gratings of FIG. 6 do not form,allowing for the continued retrieval of high fidelity holographic data.While the concentration of BHT added limits the development of suchnoise gratings when not exposed to recording light, it still permits therecording of additional holographic gratings in the medium when exposedto recording light. Thus, the addition of an appropriate amount ofretarder/inhibitor allows for high fidelity read out of theholographically stored data. Also, since the system is not changing withtime after recording the initial hologram(s), further holograms can berecorded without modifying the recording schedule, even if there is alarge time period between exposure events.

The use of polymerization retarders versus polymerization inhibitors canbe advantageous in the present invention. After the first severalexposures in recording multiple holograms, the amount of polymerizationinhibitor present in the medium can drastically be reduced. At such lowconcentrations, dark reactions may again become a problem. Conversely,with the use of a polymerization retarder, only small amounts of theretarder are reacted during any given exposure. Therefore, theconcentration of the polymerization retarder can potentially decreasesubstantially linearly and in correlation to the reduction in monomerconcentration. Thus, even late in the exposure schedule, there is enoughretarder to prevent both polymerization after an exposure andpolymerization in low light intensity areas. Effectively, thepolymerization retarder serves as a chain length limiter. Ideally, theratio of polymerization retarder to polymerizable material (e.g.,monomer) stays nearly constant throughout the exposure schedule. In sucha scenario, the chain length (degree of polymerization), potentially,stays essentially the same throughout the exposure schedule, leading toa substantially linear response for number of exposures versus timeperiod for each exposure.

The use of retarders/inhibitors/chain transfer agents is not limited toradical polymerizations. Cationic and anionic polymerizations (includingring opening polymerizations) are also subject to dark reactions. Thus,the addition of appropriate inhibitors/retarders/chain transfer agentsto these ionic chain polymerizations is also useful in reducing,minimizing, diminishing, eliminating, etc., dark reactions.

In addition to retarders, inhibitors and/or chain transfer agents,metastable reactive centers and light labile phototerminators can alsobe used to control polymerization reactions according to the presentinvention of the appropriate reactivity. For example, nitroxyl radicalscan be added as a metastable reactive center. Nitroxyl radicals createpseudo-living radical polymerizations with certain monomers. Thus, thenitroxyl radical initially behaves as a terminating agent (such as aninhibitor), however, depending on the temperature at which thepolymerization is carried out, the termination is reversible. In suchscenarios, one can both limit dark reaction and control chain length bychanging the recording temperature. Thus, it is possible to recordholograms at an elevated temperature and then cool to room temperatureto prevent further polymerizations. Additionally, it is possible torecord at room temperature, thus terminating all chains quickly like aninhibitor, and then to heat the sample to enable the addition of newphotoactive monomer to all the gratings at the same time. In this otherscenario, there is an advantage gained from the polymerization of allgratings occurring at a single time in that Bragg detuning would beuniform for all gratings involved. Other potential metastable reactivecenter include triphenylmethyl radicals, dithioesters are typically usedin Reversible Addition-Fragmentation chain Transfer (RAFT)polymerizations, that can behave as appropriate metastable reactivecenters, etc. As for ionic polymerizations, there are stable ions thatare able to perform the same function, as the example nitroxyl radicalsabove.

Use of a light labile phototerminator provides the ability to controlthe activity of the reactive species with light (as opposed to heat asdescribed above). A light labile phototerminator is any molecule capableof undergoing reversible termination reactions using a light source. Forexample, certain cobaltoxime complexes can be used to photoinitiateradical polymerizations, and yet, also terminate the same radicalpolymerizations. Dithioesters are also suitable as light labilephototerminators because they have the ability to reversibly formradicals with appropriate wavelengths of light. Under the appropriateconditions and with appropriate monomers (such as styrenes andacrylates), it is possible to restart the polymerization by irradiatingwith a photoinitiating light source (e.g., recording light). Thus, aslong as a given volume is exposed to a photoinitiating light source,radical polymerization continues, whereas when the photoinitiating lightis off or absent, the polymerizations are terminated. Metastablereactive centers and light labile phototerminators can also be used tocontrol ionic (i.e., cationic or anionic initiated) polymerizationreaction systems according to the present invention.

Yet another way to reduce, minimize, diminish, eliminate, etc., darkreactions is by the addition or inclusion of a second photoactivecompound (i.e., a photo-acid generator, photo-base generator orphoto-radical generator) that is activated by a separate wavelength thanthat used for recording holograms. For instance, if cationic ringopening polymerization is used to record the holograms, then aphoto-base generator (PBG) can be used to turn off the cationic speciesafter recording. Thus, after the desired amount of data is recorded inthe medium via the first wavelength of light, a second wavelength oflight activates the PBG which reacts with most or all of the cationicreactive sites, thus at least minimizing or preventing further cationicring opening polymerization reactions. A similar concept can be usedwith anionic and radical polymerizations where photo-acid generators andphotogenerated stabilized free radicals are used, respectively. Theconcept involving a second photoactive compound such as PBGs istypically more useful allowing the recording of some holograms at onetime, and, then recording additional holograms in the same volume at alater time until the full dynamic range is used.

For ionic chain reactions (i.e., cationic and anionic initiatedpolymerization reactions), counter ion and solvent effects can be usedto control dark reactions by terminating the reactive center. Ionicsystems are sensitive to solvent conditions because the solvent (or thesupport matrix) determines the proximity of the counter ion to thereactive center. For instance, in a nonpolar medium the counter ion willbe very closely associated with the reactive center; in a polar mediumthe counter ion may become freely dissociated. The proximity of thecounter ion can determine polymerization rate as well as the potentialfor collapse with the counter ion (depending on the counter ion used).For example, if one uses a cationic polymerization with a nonpolarsupport matrix and chloride anion as the counter ion, there is a betterprobability of terminating the reaction due to collapse of the counterion. Thus, in this way, ionic polymerizations can be terminated in acontrolled manner, since choice of support matrix and counter ionsallows one to determine the likelihood of collapse versus theprobability of propagation.

Certain monomer mixtures can also behave in a manner that can controlthe degree or rate of polymerization. For example, if a small amount ofalpha methyl styrene is present in an acrylate polymerization, theacrylate will add into the alpha methyl styrene and the styrene will notsubstantially reinitiate polymerization of the acrylate, i.e., the alphamethyl styrene retards the rate of acrylate polymerization.Additionally, the alpha methyl styrene is slow to polymerize withitself, and thus behaves as a polymerization retarder/inhibitor eventhough it is a comonomer. In the case of ionic polymerizations; using,for example, vinyl anisole in a cationic vinyl ether polymerizationresults in retarded rates of polymerization because the vinyl anisoledoes not efficiently reinitiate vinyl ether polymerization.

Because of the ability to control and particularly reduce, minimize,diminish, etc., the rate of polymerization reactions in the holographicrecording medium when the medium is not exposed to a photoinitiatinglight source (e.g., recording light), the present invention providessignificant advantages in forming at least one holographic grating inthe holographic recording medium. These include forming a plurality ofholographic gratings in the holographic recording medium, forming one ormore additional holographic gratings in the holographic recording mediumthat has previously formed therein at least one holographic grating at atime period discrete from the time period from when the additionalholographic grating(s) is formed (e.g., forming the additionalholographic gratings at least about 10 seconds afterwards to about 1week afterwards), forming the additional holographic gratings in theholographic recording medium in the same volume as the previously formedholographic grating(s), forming each holographic grating in the mediumaccording to a schedule that is a function of when the holographicrecording medium was exposed to the recording light versus the timeperiod of each exposure to the recording light, including forming eachholographic grating in the medium according to a schedule that is asubstantially linear function of when the holographic recording mediumwas exposed to the recording light versus the time period of eachexposure to the recording light, etc.

Accordingly, one embodiment of the method of the present inventioncomprises the step of forming a plurality of holographic gratings in aholographic recording medium, wherein each holographic grating in theholographic recording medium is formed by exposing the holographicrecording medium to recording light according to a schedule that is afunction (e.g., a substantially linear function) of when the holographicrecording medium was exposed to the recording light versus the timeperiod of each exposure to the recording light. Another embodiment ofthe method of the present invention comprises the step of forming atleast one additional holographic grating in a holographic recordingmedium having previously formed therein at least one holographic gratingat a time period discrete from the time period when the at least oneadditional holographic grating is formed and in the same volume thereof.Another embodiment of the method of the present invention comprises thestep of forming a plurality of additional holographic gratings in aholographic recording medium and in the same volume thereof havingpreviously formed therein a plurality of holographic gratings at a timeperiod discrete from the time period when the plurality of additionalholographic gratings are formed and wherein the plurality of additionalholographic gratings are formed according to a schedule that is afunction (e.g., a substantially linear function) of when each additionalholographic grating is formed versus the time period taken to form eachadditional holographic grating.

EXAMPLES Example 1

The following formulation is used: Formulation 1 Ingredient Wt. %1,4-Bis(2-thionaphthyl)-2-Butylacrylate 2.9 Irgacure 784 Photoinitiator0.7 WE180 Dicyclohexane Diisocyanate (from Bayer) 38.9 GlycerolPropoxylate (Mn 1000) 56.8 Tert-Butyl Hydrogen Peroxide 0.3 BHT 0.2

Formulation 1 is mixed until homogeneous and then one drop of tincatalyst is added. Matrix samples are made by placing a spacer on aglass slide, then dispensing a portion of the mixture. A second glassslide is placed on top of the dispensed mixture, and the resultingmatrix is then allowed to cure. After about 24 hours, the sample istested on a 532 nm plane wave system by writing 31 anglemultiplexed-volume holograms. Formulation 1 is stable at roomtemperature for at least 4 months and does not overdevelop holograms.

Example 2

The following formulation is used: Formulation 2 Ingredient Wt. %1,4-Bis(2-thionaphthyl)-2-Butylacrylate 3.9 Triphenylphosphine oxide(TPO) Photoinitiator 0.3 WE180 Dicyclohexane Diisocyanate (from Bayer)38.9 Glycerol Propoxylate (Mn 1000) 56.8 Nitrobenzene 0.1

One drop of tin catalyst is added to formulation 2 and matrix samplesare prepared as in Example 1. The samples are tested on a 405 nm planewave table by writing 31 angle multiplexed-volume holograms. When thenitrobenzene retarder is absent from the formulation, it is found thatthe matrix samples prepared therefrom are not suitable for high densitydata storage because the gratings become distorted due tooverdevelopment (i.e., polymerization continued after recording).

Example 3

The following formulation is used: Formulation 3 Ingredient Wt. %1,4-Bis(2-thionaphthyl)-2-Butylacrylate 4.0 TPO Photoinitiator 0.3 WE180Dicyclohexane Diisocyanate (from Bayer) 38.7 Glycerol Propoxylate (Mn1000) 56.0 2-(1,2-Ethyldiol)-Nitrobenzene 1.0

One drop of tin catalyst is added to formulation 3 and matrix samplesare prepared as in Example 1. The 2-(1,2-ethyldiol)-nitrobenzene becomespendant or attached to the matrix during the cure. The samples aretested on a 405 nm plane wave table by writing 31 anglemultiplexed-volume holograms. It is found that more photopolymerizationretarder (i.e., 2-(1,2-ethyldiol)-nitrobenzene) is needed than thenitrobenzene of formulation 2 to prevent overdevelopment of holograms.For example, when less than 1 wt % of 2-(1,2-ethyldiol)-nitrobenzene isused, overdevelopment of holograms occurs. However, at room temperature,the archival life of matrix samples prepared from formulation 3 isgreatly improved relative to those prepared from formulation 2.

All documents, patents, journal articles and other materials cited inthe present application are hereby incorporated by reference.

Although the present invention has been fully described in conjunctionwith several embodiments thereof with reference to the accompanyingdrawings, it is to be understood that various changes and modificationsmay be apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims, unless they departtherefrom.

1. A system comprising: a polymerizable component comprising at leastone photoactive polymerizable material; and a photoinitiator componentcomprising at least one photoinitiator for causing the polymerizablecomponent to polymerize to thereby form at least one holographic gratingwhen activated by exposure to a photoinitiating light source; whereinwhen a portion of the polymerizable component has been polymerized toform at least one holographic grating, the unpolymerized portion of thepolymerizable component is resistant to further polymerization when thepolymerizable component is not exposed to the photoinitiating lightsource.
 2. The system of claim 1, further comprising one or morepolymerization retarders or polymerization inhibitors so that theunpolymerized portion of the polymerizable component resists furtherpolymerization when not exposed to the photoinitiating light source. 3.The system of claim 2, wherein the one or more polymerization retardersor polymerization inhibitors comprise one or more phenols.
 4. The systemof claim 3, wherein the one or more polymerization retarders orpolymerization inhibitors comprise 2,6-di-t-butyl-p-cresol.
 5. Thesystem of claim 2, wherein the one or more polymerization retarders orpolymerization inhibitors comprise one or more nitrobenzenes.
 6. Thesystem of claim 2, wherein the one or more polymerization retarders orpolymerization inhibitors comprise a dinitrobenzene.
 7. The system ofclaim 1, further comprising one or more chain transfer agents so thatthe unpolymerized portion of the polymerizable component resists furtherpolymerization when not exposed to the photoinitiating light source. 8.The system of claim 7, wherein the chain transfer agent comprisestriethylamine.
 9. The system of claim 1, further comprising metastablereactive centers so that the unpolymerized portion of the polymerizablecomponent resists further polymerization when not exposed to thephotoinitiating light source.
 10. The system of claim 9, wherein themetastable reactive centers comprise nitroxyl radicals.
 11. The systemof claim 1, further comprising a light or heat labile phototerminator sothat the unpolymerized portion of the polymerizable component resistsfurther polymerization when not exposed to the photoinitiating lightsource.
 12. The system of claim 11, wherein the light or heat labilephototerminator comprises a cobalt complex.
 13. The system of claim 1,further comprising photo-acid generators, photo-base generators orphotogenerated radicals so that the unpolymerized portion of thepolymerizable component resists further polymerization when not exposedto the photoinitiating light source.
 14. The system of claim 13, whereinthe at least one photoactive polymerizable material undergoes a cationicring opening polymerization when activated by a first wavelength oflight, and wherein the system further comprises a photo-base generatorthat reduces the amount of cationic ring opening polymerization by theat least one photoactive polymerizable material when the photo-basegenerator is activated by a second wavelength of light different fromthe first wavelength of light.
 15. The system of claim 1, wherein thesystem uses polarity or solvation effects so that the unpolymerizedportion of the polymerizable component resists further polymerizationwhen not exposed to the photoinitiating light source.
 16. The system ofclaim 15, wherein the polymerizable component comprises a cationic oranionic initiated polymerizable material.
 17. The system of claim 1,wherein the system uses counter ion effects so that the unpolymerizedportion of the polymerizable component resists further polymerizationwhen not exposed to the photoinitiating light source.
 18. The system ofclaim 17, wherein the polymerizable component comprises a cationic oranionic initiated polymerizable material.
 19. The system of claim 1,wherein the system uses changes in photoactive polymerizable materialreactivity so that the unpolymerized portion of the polymerizablecomponent resists further polymerization when not exposed to thephotoinitiating light source.
 20. The system of claim 19, wherein thepolymerizable component comprises an acrylate and an amount of alphamethyl styrene sufficient to retard the polymerization of the acrylatewhen not exposed to the photoinitiating light source.
 21. The system ofclaim 1, wherein the polymerizable component comprises at least onephotoactive monomer.
 22. The system of claim 21, wherein the photoactivemonomer comprises an acrylate.
 23. The system of claim 21, wherein thephotoactive monomer comprises tribromophenyl acrylate.
 24. The system ofclaim 21, wherein the photoactive monomer comprises bisphenol Aglycerolate (1 glycerol/phenol) diacrylate.
 25. The system of claim 1,wherein the photoinitiator component comprises one or more of thefollowing: thionin perchlorate with a tetraphenyl borate salt as acoinitiator, triphenylphosphine oxide, andbis(7-5-2,4-cyclopentadien-1-yl)bis[2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl]titanium.26. The system of claim 1, wherein the at least one photoinitiator isactivated by radiation having a wavelength between about 200 nm andabout 800 nm.
 27. An article comprising a support matrix and thepolymerizable system of claim 1 in the support matrix.
 28. The articleof claim 27, wherein the support matrix is a thermoplastic.
 29. Thearticle of claim 27, wherein the support matrix is a thermoset.
 30. Amethod comprising the following steps: (a) providing at least oneholographic recording medium; and (b) forming at least one holographicgrating in the holographic recording medium, wherein the holographicrecording medium has a polymerizable system comprising: a polymerizablecomponent comprising at least one photoactive polymerizable material;and a photoinitiator component comprising at least one photoinitiatorfor causing the polymerizable component to polymerize to thereby form aplurality of holographic gratings in the holographic recording mediumwhen activated by exposure to recording light; wherein the unpolymerizedportion of the polymerizable component is resistant to furtherpolymerization when not exposed to the recording light.
 31. The methodof claim 30, wherein step (b) comprises forming a plurality ofholographic gratings.
 32. The method of claim 31, wherein theholographic gratings are formed in the same volume of the holographicrecording medium.
 33. The method of claim 32 wherein each holographicgrating is formed by exposing the holographic recording medium torecording light according to a schedule that is a function of when theholographic recording medium was exposed to the recording light versusthe time period of each exposure to the recording light.
 34. The methodof claim 33, wherein step (b) comprises forming the holographic gratingsaccording to a schedule that is a substantially linear function of whenthe holographic recording medium was exposed to the recording lightversus the time period of each exposure to the recording light.
 35. Amethod comprising the following steps: (a) providing a holographicrecording medium having therein one or more first holographic gratings;and (b) forming one or more additional holographic gratings in theholographic recording medium, wherein the holographic recording mediumhas a photopolymerizable system comprising: an unpolymerized portion ofa polymerizable component comprising at least one photoactivepolymerizable material; and a photoinitiator component comprising atleast one photoinitiator for causing the unpolymerized portion of thepolymerizable component to polymerize to thereby form at least oneadditional holographic grating in the holographic recording medium whenactivated by exposure to recording light; wherein the unpolymerizedportion of the polymerizable component is resistant to furtherpolymerization when not exposed to the recording light.
 36. The methodof claim 35, wherein step (b) is conducted at least 10 seconds after theone or more first holographic gratings are formed in the holographicrecording medium.
 37. The method of claim 35 wherein step (a) comprisesproviding a holographic medium having therein a plurality of firstholographic gratings in a volume thereof, and wherein step (b) iscarried out by forming a plurality of additional holographic gratings inthe holographic recording medium in the volume thereof according to aschedule that is a function of when each additional holographic gratingis formed versus the time period taken to form each additionalholographic grating
 38. The method of claim 37, wherein the holographicgratings are formed during step (b) according to a schedule that is asubstantially linear function of when the holographic recording mediumwas exposed to the recording light versus the time period taken to formeach additional holographic grating.
 39. The system of claim 2, whereinthe one or more of said polymerization retarders or polymerizationinhibitors comprise anilines.
 40. The system of claim 2, wherein the oneor more of said polymerization retarders or polymerization inhibitorscomprise sterically hindered anilines.
 41. The system of claim 2,wherein the one or more of the said polymerization retarders orpolymerization inhibitors is located on a matrix.